Focused Ion Beam (FIB) microscope systems have been produced commercially since the mid 1980's, and are now an integral part of rapidly bringing semiconductor devices to market. FIB systems produce a narrow, focused beam of charged particles, and scan this beam across a specimen in a raster fashion, similar to a cathode ray tube. Unlike the scanning electron microscope, whose charged particles are negatively charged electrons, FIB systems use charged atoms, hereinafter referred to as ions, to produce their beams. These ions are, in general, positively charged.
These ion beams, when directed onto a semiconductor sample, will eject secondary electrons, secondary ions (i+ or i−), and neutral molecules and atoms from the exposed surface of the sample. By moving the beam across the sample and controlling various beam parameters such as beam current, spot size, pixel spacing, and dwell time, the FIB can be operated as an “atomic scale milling machine,” for selectively removing materials wherever the beam is placed. The dose, or amount of ions striking the sample surface, is generally a function of the beam current, duration of scan, and the area scanned. The ejected particles can be sensed by detectors, and then by correlating this sensed data with the known beam position as the incident beam interacts with the sample, an image can be produced and displayed for the operator.
FIG. 1 is a schematic of a typical FIB system. FIB system 10 includes an evacuated envelope 11 having an upper neck portion 12 within which are located a liquid metal ion source 14 and a focusing column 16 including extractor electrodes and an electrostatic optical system. Ion beam 18 passes from source 14 through column 16 and between electrostatic deflection means schematically indicated at 20 toward sample 22, which comprises, for example, a semiconductor device positioned on movable X-Y stage 24 within lower chamber 26. An ion pump 28 is employed for evacuating neck portion 12. The chamber 26 is evacuated with turbomolecular and mechanical pumping system 30 under the control of vacuum controller 32. The vacuum system provides within chamber 26 a vacuum of between approximately 1×10 E-7 Torr and 5×10 E-4 Torr. If an etch assisting, an etch retarding gas, a deposition precursor gas, or some other reactive or non reactive gas is used, the chamber background pressure may rise, typically to about 1×10 E-5 Torr.
High voltage power supply 34 is connected to liquid metal ion source 14 and to appropriate electrodes in focusing column 16 and directing the ion beam. Deflection controller and amplifier 36, operated in accordance with a prescribed pattern provided by pattern generator 38, is coupled to deflection plates 20. A charged particle multiplier detector 40 detects secondary ion or electron emission for imaging, is connected to video circuit and amplifier 42, the latter supplying drive for video monitor 44 also receiving deflection signals from controller 36. A door 48 is provided for inserting sample 22 onto stage 24, which may be heated or cooled. Focused ion beam systems are commercially available from various companies, but the system shown in FIG. 1 represents one possible FIB system configuration.
During any beam raster operation executed by FIB system 10, which includes imaging, milling, gas assisted etching or deposition, the FIB beam deflection software and hardware deflects the beam in a preset pattern across the surface, generally referred to as rastering. At each preset location, the beam is left to dwell for a given period of time before moving to the next point in the raster. At its simplest, a raster pass consists of deflecting the beam at fixed increments along one axis from a start point to an end point, dwelling for a fixed dwell time at each point. At the end of a line, the beam waits a fixed retrace time before moving an increment in a second axis. The beam may return to the start point in the first axis and begin again, or may begin “counting down” the first axis from the point it had just reached (depending on whether the raster type is raster (the former) or serpentine (the latter). This process continues until all increments in both axes have occurred, and the beam has dwelled at all points in the scan.
It is well understood by those of skill in the art that FIB systems are used to perform microsurgery operations for executing design verification or to troubleshoot failed designs. This can involve physically “cutting” metal lines or selectively depositing metallic lines for shorting conductors together. Hence, FIB system technologies can enable prototyping and design verification in a matter of days or hours rather than weeks or months as re-fabrication would require. This FIB “rapid prototyping” is frequently referred to as “FIB device modification”, “circuit editing” or “microsurgery.” Due to its speed and usefulness, FIB microsurgery has become crucial to achieving the rapid time-to-market targets required in the competitive semiconductor industry.
While FIB microsurgery is useful for semiconductor circuit design verification, the successful use of this tool relies on the precise control of the milling process. Current integrated circuits have multiple alternating layers of conducting material and insulating dielectrics, with many layers containing patterned areas. Hence the milling rate and effects of ion beam milling can vary vastly across the device.
Unfortunately, a FIB operator is responsible for halting the milling process when a metal line of interest has been sufficiently exposed or completely cut, a process known as “endpointing”. Endpointing is done based on operator assessment of image or graphical information displayed on a user interface display of the FIB system. In most device modification operations, it is preferable to halt the milling process as soon as a particular layer is exposed. Imprecise endpointing can lead to erroneous analysis of the modified device. Older FIB systems operating on current state-of-the-art semiconductor devices do not provide image and graphical information with a sensitivity that is usable by the operator. This is due in part to the fact that older FIB systems will have imaging systems originally optimized for older generation semiconductor devices.
In particular, as semiconductor device features continue to decrease in size from sub-micron to below 100 nm, it has become necessary to mill smaller and higher aspect ratio FIB vias with reduced ion beam current. This significantly reduces the number of secondary electrons and ions available for endpoint detection and imaging. In addition, FIB gas assisted etching introduces a gas delivery nozzle composed of conductive material. This component is grounded to prevent charge build up during ion beam imaging or milling. The proximity of the nozzle to the sample surface creates a shielding effect which reduces the secondary electron detection level.
FIB operators typically rely on a real-time image of the area being milled and a graphical data plot in real time, to determine proper endpoint detection. Generally, the FIB operator is visually looking for changes in brightness of the milled area to qualitatively determine endpoint detection. Such changes may indicate a transition of the mill through different materials, such as a metal/oxide interface.
FIG. 2 illustrates an example of a problem with real-time imaging provided by current FIB systems. The FIB system generates data at each dwell point, but whether the native microsurgery software displays this data depends on a number of factors, including the particular field of view (FOV, which is directly related to the chosen magnification factor of the microscope) the FIB is operating at, the spacing of the desired increments along each axis, the dwell time per point, etc. Take for example a square mill box 60 having 0.1 μm×0.1 μm dimensions being milled at a 10 μm FOV, indicated by reference number 62. Box 62 represents the viewable area on the monitor of FIB system 10. It is noted that areas 60 and 62 are not to scale. The operator will typically elect to use such a field of view in order to properly determine the position of mill box 60 on the surface of the sample. The imaging area displayed on the user interface monitor is slightly under 1024 pixels×1024 pixels, but the FOV is divided up into exactly 1024 pixels in both the x and y axes. At this FOV, each screen pixel will be under 10 nm×10 nm, and a 0.1 μm square box (100 nm×100 nm) occupying 10 pixels×10 pixels on the screen, for 100 pixel “data points” in total that are visible to the user. If the mill box parameters are set to a spacing of 0.005 μm×0.005 μm (5 nm), then in a single pass the FIB will actually dwell at (100 nm/5 nm)+1=21 points in each box axis, generating 441 dwell points of information. Under these conditions, the FIB will display the information it generates across a region that is only 1% of the width of its display area (0.1 μm box width/10 μm FOV).
Due to the small image area and a reduced level of electron detection, the real-time image may not provide any value for the FIB operator for endpoint detection since changes in the image would be very difficult to visually detect. Furthermore, some FIB systems execute data preprocessing, such as dithering, which in fact reduces an operators' level of confidence in determining proper endpointing for state-of-the-art semiconductor devices.
To supplement the real-time imaging, a pixel intensity versus dose/depth graph is plotted in real-time during a milling operation. It is noted that both the milled image and the graph would be provided on the user monitor at the same time. FIG. 3 is a graphical plot typically relied upon by FIB operators to determine proper endpoint detection. In present example, the graphical plot is generated from the small mill box 62 in FIG. 2. The graph plots an 8-bit pixel intensity (0-255) against dose/depth, during microsurgery of a state-of-the-art semiconductor device. The plot is intended to provide the operator with an empirical indicator of endpoint detection. Typically, a transition between metal and oxide should be clearly illustrated in the plot. If the operator is to correctly determine endpoint. One difficulty in the present scheme arises from plotting the data on a fixed scale, such as the 8-bit (0-255) scale found on many instruments, that does not dynamically rescale to show more sensitive changes. A larger difficulty arises from the fact that the present system bases its analysis on the 100 pixel data points described above that result from the fact that the data is displayed on a 10 pixel×10 pixel area, even though the actual number of dwell points is 21×21=441. Thus, less than one quarter of the available data is processed by the current system, making small transitions in intensity even smaller. These two factors combine to decrease the sensitivity of the present system.
Accordingly, there is a lower probability of successful endpoint detection by the FIB operator, and a lower probability of successful design verification, leading to no data on the success of a new design, or worse erroneous results that may lead to incorrect design changes being propagated into the final device. New FIB systems calibrated for state-of-the-art devices may be available, but the cost of replacing older FIB systems is prohibitive. Modification of the FIB system firmware or hardware is generally prohibited, since FIB system modification voids certification of the system and any manufacturer warranties.
In SIMS (secondary ion mass spectrometry) systems for analyzing or detecting chemicals in a semiconductor material, software has been developed for enhancing images to highlight areas where a specific chemical is detected. However, SIMS operation does not require endpointing. In fact, a SIMS operator can arbitrarily determine when the process should stop without any concern for what layers of material have been destroyed by the ion beam. In Microsurgery, there is typically a requirement that the device remain electrically active after the circuit editing has been performed. In SIMS, not such requirement exists, and in fact it is often a requirement that SIMS be performed on a device that is not complete (typically just a bare silicon die with no conductor layers fabricated) and therefore cannot operate. In fact, for semiconductor devices, the emission of particles from destroyed structures required for determining their chemical compositions. In contrast for example, FIB endpointing operations require precise control over when the mill should stop so as not to destroy a specific structure.
Additionally, as devices become more complex, the number of operations required to perform successful microsurgery typically increases, as do the complexity of these operations. It is generally required and always desirable that all elements of the microsurgery (i.e. cuts of certain interconnect and joins between other interconnect) function correctly. After a microsurgery has been performed, it is necessary to test the device to determine if the design change implemented by the microsurgery has had the desired result. The method of developing test programs and test systems is well known in the industry. If the device does not function as expected, it is not uncommon for the test systems to produce data that can be analyzed to determine what is not functioning correctly.
The difficulty arises in more complex circuit edit tasks, in determining if the unexpected function is due to a flaw in the design or an error in how the design was implemented through microsurgery. At present, the microsurgery operator must rely on a few images digitally captured during the course of the circuit edit, and their own memory as to what might have gone wrong that could have produced the result observed. As time passes and other microsurgery tasks are performed it is difficult for the operator to recall every element of a particular surgery, and even if the test results are obtained very quickly after completion of the surgery, it is not possible for all operators to recall all aspects of the task.
It is, therefore, desirable to provide a method for improving endpoint detection in FIB systems.